The invention disclosed and claimed herein generally pertains to apparatus for substantially reducing the acoustic noise or disturbance which is experienced by patients, users or others in the vicinity of a magnetic resonance (MR) imaging system. More particularly, the invention pertains to apparatus of the above type wherein MR imaging is carried out by means of an MR imaging system or scanner provided with a bore, or internal imaging volume, and the apparatus includes acoustic absorbing or barrier material disposed for insertion into the bore. Even more particularly, the invention pertains to apparatus of the above type which is selectively removable from the MR system, so that it may or may not be used for a particular imaging procedure, as determined by the system operator.
As is well known by those of skill in the art, MR imaging systems employ electrically excited coils to impose time varying magnetic fields on the static primary B0 field produced by the system""s main magnet. The imposed fields have associated currents which flow through conductors. Since these conductors occur within a magnetic field, corresponding forces are applied to the conductors, which cause dynamic motions to be propagated through the MR system. Moreover, typical current waveforms contain repetitive pulses with fast transitions that produce vibrational energy within the audio frequency range. This causes the MR imaging system or scanner to radiate acoustic or sound pressure waves, which may be very disturbing to both patients and system operators, and may interfere with their communications. Also, very high speed scans in high field main magnets can generate noise levels which exceed acceptable exposure levels.
In the past, various schemes have been proposed to reduce the acoustic noise which is experienced by patients and others located in and around an MR scanner. However, such schemes have generally met with limited success, at best, due to severe constraints on materials which may be used within an MR scanner, and within the imaging bore. Acquisition of usable MR image data from a patient is an extremely sensitive procedure. Accordingly, no materials can be used which would distort the magnetic field waveforms required to generate MR data signals in a patient, or which would interfere with reception of the data signals by the receive coil of the MR scanner. Further constraints on acoustic noise reduction are imposed by the very limited space which is available within the coils of the scanner. This space determines the maximum diameter of the bore, or imaging volume.
The invention is generally directed to comparatively simple and inexpensive apparatus for reducing the level of acoustic noise or energy which is received by an imaging subject located within the bore of an MR imaging system. The apparatus comprises a bridge fixed within the bore to support the subject during MR data acquisition, and further comprises a frame which is removably insertable into the bore for placement upon the bridge. The frame, when in place upon the bridge, defines a space within the bore which is disposed to receive the imaging subject. The frame is covered by a layer of material which is capable of absorbing substantial amounts of acoustic energy, in the audio frequency range, which is present in the bore. At the same time, the material is highly transmissive to RF signals required for MR data acquisition and likewise avoids spurious RF signal generation which would add noise to MR data.
In a preferred embodiment of the invention, the frame comprises a rigid, selectively curved shell provided with parallel spaced-apart edges, which extend along the bridge when the shell is in place thereupon. Means joined to each of the edges of the shell are positioned between the shell and the bridge, when the shell is in place thereupon, for isolating the shell from mechanical vibrations occurring in the bridge. For example, each of the isolation means may comprise a strip of compliant material. Usefully, the shell is formed of a material such as fiberglass, and the acoustic energy absorbing material comprises open cell foam which is joined to the upper surface of the curved shell. The shell has a cross section comprising a circular arc of specified size, such as 220xc2x0.
It is anticipated that an embodiment of the invention would be particularly useful in imaging smaller patients, such as infants, children and certain elderly persons. Such patients would require less space in the imaging bore, so that correspondingly more space would be available to accommodate the frame and the acoustic absorbing material supported thereby. It is anticipated further that the invention may be embodied in a form which is highly portable, so that the embodiment of the invention may or may not be used for a particular MR imaging procedure, as determined by the system operator.